It is known that lubricating oils and in particular, the oils for gear case designed for internal combustion engines contain various additives that serve to improve the performances of the oils while they are being used. Certain additives are used for increasing the viscosity index of the oil while others ensure, for example, keeping the gear case clean by suspending the insoluble deposits, also called slime, that form in the oil.
The additives intended to improve the viscosity index of the oil must have, on one hand, a sufficient thickening effect on a light lubricating oil in order that at elevated temperatures, the lubricating properties of this oil correspond to those of a heavier lubricating oil and, on the other hand, a limited thickening effect on a light lubricating oil in order that at lower temperatures, the characteristics of the oil not be disturbed at those low temperatures. Those are generally polymeric compounds with long chains such as, for example, polyisobutenes, polymethacrylates, polyalkylstyrenes, partially hydrogenated copolymers of butadiene and of styrene and amorphous copolymers of ethylene and of propene. The additives that ensure the maintenance of the cleanliness of the gear case are, on the one hand, detergents such as, for example, sulfonates, phenates or organic phosphates of polyvalent metals which are particularly effective when the engine is hot and, on the other hand, dispersants such as, for example, succinimides or succinic esters alkylated by a polymer of isobutene or of propene on the carbon atom in alpha of the carbonyl of the succinimide, these additives working more particularly during travel when the engine is cold, such as those automobiles that make frequent stops.
There has also been proposed additives for lubricating oils intended to ensure simultaneously the improvement of the viscosity index of the oils and the dispersion of the slime they may contain. Those compounds are obtained, for example, by introducing monomers that have been nitrogenized, either by copolymerization or by grafting, on a thickening polymer. The nitrogenized dispersing monomers are preferably selected among the maleimides, vinyl imidazolines, vinyl pyrrolidines, vinyl pyridines or N,N-dialkylaminoethyl methacrylates. The following patents claim the use of such compounds:
EP Nos. 171,167; 167,195; 164,807; 145,369; U.S. Pat. Nos. 4,606,834; 3,732,334; and DE No. 874,068.
However, multifunctional additives for lubricants having other properties than improvers of viscosity and dispersants have been sought. The formulators are increasingly seeking additives having a great thermal stability and a resistance to oxidation which are important because of the increasing severity of the functioning conditions of engines. Thus, there is observed an increase of the working temperature of the lubricant, particularly in diesel engines subjected to strong charges where the temperature at the top of the piston, in the vicinity of the first recess, can reach and actually exceed 350.degree. C. Lubricants have a tendency to oxidize when they are subjected to elevated temperatures. This decomposition is proportional to the thermal stability and to the resistance to oxidation of the oil and the additives. It causes the formation and deposit of resinous compounds and carbonaceous substances on the walls of the piston. If these deposits become too large, they can start a gumming or abrasion of the piston and increase the wear of the metal parts in motion. Therefore, it is important to control and limit such deposits by using additives that are effective and stable at elevated temperatures.
In the prior art, the protection of the lubricating oils by antioxidizing organic molecules has been extensively described. These compounds are, in general, distinguished by the manner in which they act; on one hand, the inhibitors of the free radicals (phenols, aromatic amines, phenothiazines and the like) and, on the other hand, the products that destroy the peroxides (metal dithiophosphates or dithiocarbamates).
Likewise, known are the metal deactivators (benzothiazoles, tetrazole and the like), which protect the metals against the action of the oxygenated compounds formed during the degradation of the lubricants. These compounds are not antioxidants in the classic sense of the word, but metal anticorrosives or passivators which bring a supplementary protection in respect to the corrosive products formed by oxidation of the oil.
The use of antioxidizing molecules of low molecular weight can involve several inconveniences; in particular, these products have a limited solubility in oil and are, at times, volatile and subliminal when the engine works at a rate close to its maximum rate.
These inconveniences can be eliminated by using polymers that improve the viscosity and provide antioxidizing functions. Another advantage of these multifunctional polymers is to concentrate the antioxidizing structures carried by the polymer in the upper parts of the engine during the "flash" evaporations of the oil when the protection against oxidation is most necessary.
The polymers that improve the viscosity and are antioxidizing were known in the prior art.
U.S. Pat. No. 4,500,440 claims an ethylene-propylene copolymer carrier of epoxide functions obtained by grafting glycidyl methacrylate and treated by amino-5-tetrazole.
U.S. Pat. Nos. 4,339,561 and 4,281,192 and DE No. 2,943,166 describe copolymers of alkyl (meth)acrylates and of hydroxy-2,5-trimethyl-3,4,6 benzyl (meth)acrylamide and eventually of a dispersant monomer such as N,N, dimethyl aminoethyl (meth)acrylate.
U.S. Pat. No. 4,170,562 concerns the reaction of olefinic polymers oxidized with a base of Mannich starting from a polyamine, an aldehyde and a phenol. This reaction gives an ill-defined mixture of several products.
U.S. Pat. No. 4,668,412 concerns the reaction of maleic anhydride copolymers and methacrylic esters with Mannich bases; but, it is known that the copolymerization of maleic anhydride is very incomplete and the copolymer formed is accompanied by a monomer hard to separate. The formation of a Mannich base without the addition of a supplementary amine furnishes a cross-linked product that cannot be used.
U.S. Pat. Nos. 4,205,151 and 4,078,091 describe homopolymers of a maleimide prepared by reaction of a disubstituted paraaminophenol with maleic anhydride.
U.S. Pat. No. 4,098,709 mentions the preparation and use of antioxidizing polymers by reaction of a phenolic derivative on a polymer carrier of free carboxylic acid functions. The product having an ester-amide structure is sensitive to hydrolysis. The synthesis of the phenolic pattern is long and costly. It passes through four stages and numerous purifications.
U.S. Pat. Nos. 4,096,139 and 3,951,831 concern polymers prepared by postesterification of a polymer carrier of free carboxylic acid functions by a beta hydroxy amide replaced by a derivative of the benzothiazole.